General Description
1.1 Introduction
The MC68HC05P4A is a 28-pin MCU (microcontroller unit) based on the MC68HC05P4. The memory map includes 4160 bytes of user ROM and 176 bytes of RAM. The MCU has two 8-bit input/output (I/O) ports, A and C. Port B has three I/O pins and port D has two pins, one that is I/O and the other input only. The MC68HC05P4A includes a simple serial I/O peripheral (SIOP) and an on-chip mask programmable computer operating properly (COP) watchdog circuit.

1.2 Features
Features of the MC68HC05P4A include:
• Low cost
• HC05 core
• 28-pin package
• On-chip oscillator with RC (resistor capacitor) or crystal/ceramic resonator mask options
• 4160 bytes of user read-only memory (ROM), including 16 user vector locations
• ROM security feature(1)
• 176 bytes of on-chip random-access memory (RAM)
• 16-bit timer
• 20 bidirectional input/output (I/O) lines, one input-only line
• Mask programmable keyscan (pullups and interrupt) on eight port pins (PA0–PA7)
• Two port pins with high current drive capability
• User mode
• Self-check mode
• Power-saving stop and wait modes
• Edge-sensitive or edge- and level-sensitive interrupt trigger mask option
• Simple serial I/O port
• Mask option selectable computer operating properly (COP) watchdog timer

1.3 Mask Options
The MC68HC05P4A has 13 mask options:
• CLOCK, RC or crystal
• IRQ, edge-sensitive only or edge- and level-sensitive
• SIOP, most significant bit (MSB) or least significant bit (LSB) first
• COP watchdog timer, enable/disable
• Keyscan pullups and interrupts on port A, enable/disable by pin
• STOP instruction